Filtration systems have been used to clarify gas streams in numerous applications. Typical filtration systems employ a modular concept in which a plurality of short filter elements are connected end to end to form a stack of filter elements and then installed into a filter housing. The influent gas streams are pumped into the housing and clarified by allowing them to pass through the filter elements.
Since the release of any airborne radioactive particulate matter in the nuclear industry is impermissible, clarification of off gas streams and ventilation air in nuclear applications is an especially critical filtration application which requires high efficiency gas filters including high efficiency particulate air (HEPA) filters and ultra low penetration air (ULPA) filters. The high efficiency gas filters are designed to have sufficient medium thickness and fine fibers and are sized at an appropriate flow rate per unit of filter area to effectively retain virtually all of the particulate matter in the gas streams. By specification, HEPA rated filters exhibit a particle removal efficiency of 99.97% when challenged by thermally generated monodisperse DOP smoke particles with a diameter of 0.3 micron. Similarly, ULPA rated filters exhibit a particle removal efficiency of 99.999% when challenged by thermally generated monodisperse DOP smoke particles with a diameter of 0.3 micron.
Typical high efficiency gas filters, including, for example, glass fiber filters are mechanically weak and fragile so that they may be structurally damaged when subjected to high air flow, high temperature, high humidity, heavy dust loads or combinations of these factors.
Recently, a novel high efficiency metal filter, called Pall UltraMet Air Filters made by Pall Corporation, has been developed for use in the clarification of gas streams. The metal filter material is a rugged, fibrous sinter bonded medium which is relatively thin and pleated so that high flows can be handled at low pressure differentials in compact assemblies. The high efficiency metal fiber filter has a high surface area, high voids volume medium, exceptional dirt holding capacity, and high mechanical strength.
During the filtration operation, the direct interception of the solids by the filter medium typically results in the formation of a permeable cake of the larger bulk solids on the surface of the filter medium. The dirt capacity of the filter is also consumed when quantities of particulate matter, which are typically smaller than the pore size of the medium and filter cake, are trapped within the interior of the filter medium by the van der Waals forces and electrostatic forces between the medium surface and the particulate matter. The particulate matter effectively clogs the filter elements, increases the pressure drop across the filter, and interferes with the filtration operation. In a typical ventilation stream, for example, the circulating pressure is approximately 20 inches of water pressure so that even the small increases in the differential pressure across the filter medium caused by the small particles will adversely affect the system operation.
The conventional glass fiber filters and sand bed filters have been largely of the disposable type because attempts to clean the filters, including the use of expensive and caustic chemical or acid baths requiring exposure and handling of radioactive waste material, have been largely unsuccessful. Unlike these conventional high efficiency filters, it is impractical and undesirable to dispose of the high efficiency metal filters when they are clogged because they are relatively expensive and the environmental advantage of a regenerable system is lost. Unfortunately, attempts to clean the metal filter using a blowback cleaning method in which a reverse flow of air is forced through the filter element have not been entirely effective because the blowback method is unable to purge the smaller particulate matter trapped within the matrix of the filter.